The development and growth of non-contact type IC cards have been remarkable. IC card functions etc. are now being built into mobile phones due to the card size. Non-contact type IC cards are disclosed in for example Patent Documents 1 and 2.
FIG. 1 is a diagram showing the configuration of a front end circuit 10 of a general non-contact type IC card use R/W (reader/writer) device.
The front end circuit 10 of this non-contact type IC card use R/W device is mainly configured by, as shown in FIG. 1, a reception side circuit 11, a transmission side circuit 12, a serial resonance use capacitor 13, an R/W antenna 14, etc. A resistor 15 is an internal resistor of an (antenna) coil 16.
Further, in FIG. 1, C1 indicates a capacitance of the capacitor 13, R1 indicates a resistance value of the resistor 15, and L1 indicates an inductance of the coil 16.
The operation of the front end circuit 10 of the R/W device of FIG. 1 will be explained.
At the time of the transmission of data, a modulation wave signal is output from the transmission side circuit 12, a current flows in the capacitor 13 connected in series to the antenna 14, and the coil 16 forming the antenna 14, and a magnetic field is emitted from the antenna 14.
On the other hand, at the time of reception, a constant carrier signal is output from the transmission side circuit 12, but it is possible to perform load modulation on the card side so that the carrier signal received at the reception side circuit 11 becomes a load modulated signal and the demodulation data can be extracted.
In the transmission and reception of data, the capacitor 13 and the coil 16 form a resonance circuit. At this time, an impedance seen from the transmission side circuit 12 becomes small (much current flows), and the transmission magnetic field from the antenna 14 becomes the largest.
Namely, the front end circuit 10 at the time of the transmission/reception is configured with the coil 16 and the capacitor 13 connected in series. Further, basically ideally a carrier frequency of the signal used is set so as to coincide with a resonance frequency of the coil 16 and the capacitor 13 and the impedance is the lowest at the resonance frequency.
The coil 16 configuring the loop antenna is expressed by the resistance 15 including the loss and forms a serial resonance circuit together with the capacitor 13.
The resonance frequency f0 thereof is expressed as in the following equation.
[Equation 1]f0=1/(2π*√(L1*C1))  (1)
An impedance Z of the antenna circuit at this time becomes as follows. When a resistance value R1 of the resistor 15 is small, the current can flow in the antenna with a high efficiency and the generated magnetic field becomes the maximum.
                    [                  Equation          ⁢                                          ⁢          2                ]                                                                                                Z              =                            ⁢                                                R                  ⁢                                                                          ⁢                  1                                +                                  j                  ⁢                                                                          ⁢                  ω                  ⁢                                                                          ⁢                  L                  ⁢                                                                          ⁢                  1                                +                                  1                  /                                      (                                          j                      ⁢                                                                                          ⁢                      ω                      ⁢                                                                                          ⁢                      C                      ⁢                                                                                          ⁢                      1                                        )                                                                                                                          =                            ⁢                              R                ⁢                                                                  ⁢                1                                                                        (        2        )            
Next, the front end circuit used in the non-contact type IC card will be explained.
FIG. 2 is a diagram showing the configuration of principal parts of the front end circuit used in a non-contact type IC card.
The front end circuit 20 used in the non-contact type IC card is principally configured by, as shown in FIG. 2, a transmission and reception side circuit 21, a capacitor 22, a card antenna 23, a resistor 24, etc.
Further, in FIG. 2, C2 indicates the capacitance of the capacitor 22, R2 indicates the resistance value of the resistor 24, and L2 indicates the inductance of the coil 25.
In the circuit of FIG. 2, the card function is set so that the resistor 24 and the coil 25 are connected in series, a capacitor 22 is connected parallel to the two ends of these, and resonance occurs at the used frequency.
Further, at the time of the resonance, basically ideally, the impedance of the combined circuit of the card antenna 22 and the parallel connected capacitors 22 becomes the highest.
The parallel resonance frequency f0 is expressed by the following equation.
[Equation 3]f0=1/(2π*√(L2*C2))  (3)
The impedance Z of the antenna circuit at this time is expressed by the following equation.
[Equation 4]Z=1/(1/(R2+jωL2)+(jωC2))  (4)
The impedance Z expressed by (Equation 4) where R2 is small becomes as follows, and the reception voltage becomes the maximum at the peak of the impedance.Z≈∞  [Equation 5]    [Patent Document 1] Japanese Patent Publication (A) No. 2002-334310    [Patent Document 2] Japanese Patent Publication (A) No. 2004-355212